The field of the invention is insulating bodies and more particularly bodies of high dielectric strength and methods for making such bodies.
As is known, the dielectric strength of a material corresponds to the energy necessary to break the internal bonds of the material. Thus, the greater the dielectric strength of a material, the greater the electric field required to break the internal bonds of the material.
In the case of a material which is to be subjected to an electric field, two parameters are determinative: the local electric field, which depends on the average applied field, the geometry of the material in the electric field and the possible presence of inhomogeneities (impurities, voids, etc.) increasing the field locally; the bond energy of the molecular bonds of the material. The weakest bonds may be broken by application of the electric field and degradation of the material then results.
In the case of materials based on polymers, the weakest bonds are the Van der Waals bonds between the molecules of the polymers. The energy of these bonds depends essentially on the distance between the molecules. The greater the distance, the less the energy.
In polyethylene for example, dielectric breakdowns occur mainly in the amorphous regions, i.e. those outside the crystalline structure, when the cohesion of the material is weakest, whereas the crystalline regions are better able to resist, thanks to their smaller inter-molecular distance.
Thus the dielectric strength of a polymer, that is to say the ability to withstand electric fields, increases with the organization of the polymer structure and with reduction in the distance between the molecules, these two factors being related.
Materials with high dielectric strength can thus be obtained when their manufacture is effected by drawing the materials. This explanation is to be found in particular in the technical journal IEEE Trans. EI No. 22 (5) p. 573, 1987.
The manufacture of polymers takes place either in the molten state or at a temperature above the glass transition temperature, at which molecular movement occurs. The characteristics of the end product depend on the method of manufacture, in particular on the strength of strains applied to the molten polymer. The stresses applied to the polymer organize the structure of the final product and can be of two types: shearing and extension. For each type of stress the resulting local strain is related to the ratio between the stress and the viscosity of the polymer: EQU GV=C/V (1)
where
GV is the velocity gradient between two points of the polymer. PA0 C is the applied stress. PA0 V is the viscosity of the polymer. PA0 mixing the polymer materials; PA0 heating the material resulting from the mixture of the polymer materials; PA0 applying a stress to the resultant material in such a manner as to organize it. PA0 making a synthetic material having at least two peaks of different molecular concentrations; PA0 heating the synthetic material; PA0 applying a stress to the synthetic material in such a manner as to organize it.
Thus, for a given applied stress, the organization of the polymer is inversely proportional to its viscosity. High dielectric strength of the material is thus obtained when its viscosity is low. Conversely, a high viscosity of the material leads to a low dielectric strength of the polymer.
However, in the case of manufacture of the material by extrusion, for example in making the insulation of an electric cable, the overall viscosity of the material at the temperature of manufacture should be high enough to avoid significant deformation of the insulator during the cooling phase of the cable. Thus, as shown in FIG. 1, showing a cross-section of an electric cable emerging from an extrusion machine sheathing an electric conductor 10, also known as an extruder, the polymer 11 insulating the conductor 10 has a tendency to flow in the direction 12 by reason of gravitational force, when the polymer does not have high enough viscosity. The result is that the conductor 10 is eccentric relative to the insulator 11 and the quality of shape of the extruded object is no longer preserved.
It is therefore not known to make products with high dielectric strengths based on polymers by extrusion, since there is a contradiction between a high dielectric value of the object, requiring low viscosity, and a quality of shape of the extruded object, requiring a high viscosity.
The object of the present invention is in particular to provide a method of making such extruded products which reduces this problem.
More especially, it is an object of the invention to provide a method whereby polymer products can be made so as to exhibit both a good quality of shape and a high dielectric strength.
Another object of the invention is to provide such products which both exhibit good geometrical properties and a high dielectric strength.